PHOTOKEACTIVATION 459 



as the sole carbon source, in which the bacteria have grown to complete 

 exhaustion of the sugar. If, on the contrary, the bacteriophages are 

 adsorl^ed on l)a(^teria in a medium supporting growth, the photoreactiv- 

 ability of the phages decreases with time elapsed between infection and 

 exposure to the photoreactivating light. For example, for bacteriophage 

 T2 adsorbed on E. coli B at 37^C in beef extract, very little or no photo- 

 reactivation can be obtained if exposure to white light is started 30 min 

 after infection. 



Phage inactivation is obtained also by irradiating bacteria infected 

 with active bacteriophages (Luria and Latarjet, 1947); also in this case 

 photoreactivation can be obtained (S. Benzer, personal communication). 

 Photoreactivation is also obtained if the inactive phage is adsorbed on 

 sensitive bacteria previously irradiated with ultraviolet. 



3-4. COMPARISON OF SURVIVAL CURVES AFTER ULTRAVIOLET 



TREATMENT IN PRESENCE AND IN ABSENCE 



OF PHOTOREACTIVATION 



Survival curves (see Sect. 3-1) of different phages in absence of photo- 

 reactivation are of two types: for the large phages (T2, T4, T5, TG) they 

 have an initial curvature with dow^nward concavity and then tend to 

 straight lines; for the small phages (Tl, T3, T7) they are straight near 

 the origin and then decrease in slope, tending again to straight lines 

 (Dulbecco, 1950). 



Survival curves obtained after the irradiated samples have been 

 exposed to photoreactivation have similar shapes as the curves in dark- 

 ness, but they are shifted in the direction of a higher survival. 



The comparison of the curves obtained for a given phage in darkness 

 with those obtained after maximum photoreactivation shows that if the 

 ultimate straight parts of the two curves are extrapolated back toward the 

 origins, they intersect in the proximity of the abscissa. For the larger 

 phages, which have a curvature with downward concavity, the inter- 

 section point lies to the right of the origin. For the small phages it lies to 

 its left (Fig. 12-1) (Dulbecco, unpublished). 



The reason for this behavior is not yet clear. In the section on bacterial 

 photoreactivation (Sect. 5-3) it will be shown that in bacteria a different 

 relation between survival curves in dark and after photoreactivation has 

 been observed, a relation which has been called the "principle of constant 

 ultraviolet dose reduction." It should be emphasized here that the same 

 relation does not hold in bacteriophage reactivation. The methods for 

 determining the existence of a constant dose reduction will be discussed in 

 Sect. 5-3, where the explanation of Fig. 12-1, in which the two tests for 

 such a determination have been applied to phage, both with negative 

 results, is also given. 



For every phage the survival curves in darkness and after maximum 



